mhequ – Multicolumn equations, tags, labels, sub-numbering

MHequ sim­pli­fies cre­at­ing multi-col­umn equa­tion
en­vi­ron­ments, and tag­ging the equa­tions therein. It sup­ports
sub-num­bers of blocks of equa­tions (like (1.2a), (1.2b), etc)
and ref­er­ences to each equa­tion in­di­vid­u­ally (1.2a) or to the
whole block (1.2). The la­bels can be shown in draft mode.